1. (Related Art)
In recent years, along with advances in electronic devices such as semiconductors and advances in digital control technologies utilizing microcomputers and the like, digital control with microcomputers are becoming the mainstream of drive control of pointers in pointer instruments for vehicles, such as speedometers, instead of conventional mechanical control.
A pointer instrument can display an analog quantity such as a velocity by a position of a pointer swinging on a scale plate, which is intuitively, understood. For digitally controlling the swing position of the pointer, a stepping motor is coming into use. In this case, the stepping motor is used for positioning of the pointer by open-loop control. Specifically, rotating driving pulses in a forward direction or a backward direction are applied to the stepping motor of which a rotational output shaft is joined to the pointer, so as to effectuate two-way travel of the pointer and to control a travel distance of the pointer according to the number of the rotating driving pulses. Since the amount of rotation (the angle of rotation) of the stepping motor can be uniquely set by the number of the rotating driving pulses, it is possible to position the pointer and the like without adoption of complicated feedback control using a position sensor.
As disclosed in U.S. Pat. No. 5,959,378, for example, such a stepping motor comprises a rotor including a permanent magnet having multiple poles magnetized to be reversed alternately along the circumferential direction, stator yokes each being formed into a bent shape so that two ends thereof respectively constitute magnetic pole portions facing closely to a magnetic pole-passing surface of the permanent magnet, and through-hole shaped winding bobbins on which coils for exciting the stator yokes are pre-wound, and the stepping motor has a structure that intermediate portions of the stator yokes are inserted into the winding bobbins. This stepping motor is usually combined with a deceleration mechanism composed of a plurality of gears and is used for the pointer instrument and the like.
2. (Existing Problems)
The foregoing stepping motor has characteristic and structural problems as described below.
First, regarding the characteristic problems, although the above-described stepping motor generates magnetic attraction between the permanent magnet and the stator yokes, this magnetic attraction is also generated in a non-excited state when electricity is turned off. Further the magnetic attraction differs largely depending on the rotating position (angle) of the rotor. Specifically, there is generated the magnetic attraction which differs largely depending on a relative position between the magnetic pole of the permanent magnet forming the rotor and the magnetic pole portion of the stator yoke. This magnetic attraction generates force to pull the rotor to a specific rotating position even in the non-excited state (when the electricity is turned off), what is called detent torque.
Whereas this detent torque brings an advantage to enable the rotor to be stopped in a position at a given angle even in the non-excited state without provision of a special mechanism, the detent torque also incurs torque ripple in a rotational output of the stepping motor. This torque ripple interferes continuous and smooth rotation and also causes vibration and noise. Further, since the torque ripple is excessively high in the conventional stepping motor, there was a problem that continuous and smooth rotation could not be obtained, and vibration and noise were large.
Therefore, if the conventional stepping motor is used for driving the pointer of the above-described pointer instrument, for example, then there occurs a problem that movement of the pointer is not smooth and therefore the pointer cannot indicate an analog quantity such as a velocity without discomfort. Moreover, since the torque ripple also causes vibration and noise, a problem also occurs that the conventional stepping motor is not suitable for use in a place where silence is required.
This type of stepping motor is frequently used together with a plurality of gears for transmitting the rotation of the rotating shaft of the motor while decelerating the rotation. In this case, a problem also occurs that the noise attributable to the vibration caused by the torque ripple is amplified by the gears and engagement of the gears becomes unstable due to the vibration.
As the related art aiming at reduction in the detent torque, the specification of U.S. Pat. No. 5,959,378 provides the technology of forming the shapes or the magnetic poles of the stator yokes, particularly the surfaces facing close to the magnetic pole-passing surface of the permanent magnet that forms the rotor, in unequal spaces with respect to the magnetic pole passing surface. However, sufficient reduction in the detent torque has turned out to be practically difficult with forming the shapes of the magnetic poles of the stator yokes alone. Moreover, when the shapes of the magnetic poles of the stator yokes were formed as described above, it became clear that other problems were also incurred, such as reduction in rotational torque attributable to an increase in an effective gap between the magnetic poles of the permanent magnet and the magnetic poles of the stator yokes.
As a countermeasure for the noises, it is effective to incorporate the entire motor inclusive or the gears into a plastic casing. The plastic casing is suitable for mass production by molding and is therefore advantageous to cost reduction. Moreover, in the case of an electrically insulative plastic casing, other advantages such as capability of simplifying an insulative structure of motor feeding terminals. However, the plastic casing has more difficulty in dimensional accuracy as compared to that of metal, and the degree of deformation attributable to thermal expansion or the like is also large. Accordingly, distances between pitches of gears tend to fluctuate and smooth transmission of rotation is from time to time inhibited due to occurrence of abnormal engagement such as play between the gears. In some cases, rotation may not be transmitted accurately or there may be even a risk of disability of rotation due to disengagement of the gears.
Next, regarding the structural problems, as described previously, this type of stepping motor comprises the rotor including the permanent magnet having the multiple poles magnetized to be reversed alternately along the circumferential direction, the stator yokes each being formed into the bent shape so that two ends thereof respectively constitute the magnetic pole portions facing close to the magnetic pole-passing surface of the permanent magnet, and the through-hole shaped winding bobbins on which the coils for exciting the stator yokes are pre-wound, and the stepping motor has the structure that the intermediate portions of the stator yokes are inserted into the winding bobbins.
In the above-described structure, although the intermediate portions of the stator yokes are inserted into the through-hole shaped winding bobbins on which the coils for exciting the stator yokes are pre-wound, the stator yokes of the bent shapes cannot be Inserted into the winding bobbins directly. Therefore, the stator yoke was conventionally preformed as two-divided members in the longitudinal direction. Then, one of the divided yoke members was inserted into the winding bobbin, and the relevant divided yoke member and the other divided yoke member were joined together by welding outside the bobbin, thus eventually forming the stator yoke integrated as the bent shape. Alternatively, the magnetically integrated stator yoke of the bent shape was assembled by stacking multiple yoke pieces being divided respectively in the longitudinal direction and in the thickness direction.
However, when the stator yoke is assembled by welding, a problem occurs that magnetic characteristics of a magnetic material constituting the stator yoke vary due to such welding, whereby a given function as the stator yoke cannot be obtained. As the material for the stator yoke, it is necessary to use toe material which guarantees given magnetic characteristics. However, if a process such as welding is performed thereon, such a guarantee cannot be maintained.
It is desirable to provide the stator yoke with plating for protection such as rust-proofing. However, if the plated stator yoke is subjected to the welding process, the processed portion loses protection by plating and a different kind of alloy is formed therein, thus a problem occurs that quality such as rust-proofing cannot be guaranteed.
The problem attributable to welding is avoidable when the stator yoke is assembled by stacking multiple yoke pieces. However, a problem occurs that contact areas and contact conditions at the overlapping portions of the respective yoke pieces become uneven and the given magnetic characteristics cannot be obtained in good reproducibility, and that magnetic fields respectively created by edge portions of the respective yoke pieces adversely affect other yokes. Moreover, since the multiple yoke pieces are used in this case, there is also a problem that the number of components is increased and the respective components tend to separate apart easily, whereby workability of assembly is significantly reduced.